Amplifiers are widely used in the prior art for amplifying input signals applied thereto to provide amplified output signals. This is particularly important in radio receivers in which radiation received thereat generates corresponding antenna received signals which typically have an amplitude of microvolts. The radio receivers employ amplifiers therein to amplify such received signals to an amplitude in the order of multivolts to volts, for example to drive a loudspeaker. Since it is difficult to prevent amplifiers designed to amplify at radio frequencies from spontaneously oscillating, especially if they comprise cascaded gain providing stages, it is customary to heterodyne the received signals to lower intermediate frequencies whereat it is easier to provide a high degree of amplification and also provide more selective bandpass signal filtration.
In prior art radio receivers, it is therefore customary to provide a majority of signal amplification required at intermediate frequencies, namely frequencies lying intermediate between that of the radiation received and audio or video frequencies. For example, a radio receiver receives radiation at a frequency of 500 MHz and generates a corresponding antenna received signal also at 500 MHz. The receiver heterodynes the received signal to generate an intermediate frequency signal in a frequency range around 10.7 MHz which is then amplified and filtered, and finally demodulates the amplified intermediate frequency signal to generate a corresponding audio output signal having signal components in a frequency range of 100 Hz to 5 kHz.
Recently, because the radio frequency spectrum is becoming increasingly congested, there is a trend to use an ultra high frequency (UHF) range in contemporary communications systems, namely around 500 MHz; transmission at microwave frequencies, for example 1 GHz to 30 GHz is now also employed. Associated with this is a trend in modern radio receiver design to employ intermediate frequency amplification at several tens of MHz or greater, this is done in order to obtain adequate ghost image rejection associated with using heterodyne processes.
In modem  modern mobile phones, most signal amplification is provided in intermediate frequency amplifier circuits incorporated therein. These circuits comprise transmission amplifiers and associated surface acoustic wave (SAW) or ceramic filters to provide a narrow bandpass signal amplification characteristic; the circuits and their associated filters are conventionally referred to collectively as an “intermediate frequency strip”. Such transmission amplifiers consume significant power in operation, for example intermediate frequency amplifier circuits employed in mobile telephones typically consume between several hundred microamperes and several mA of current when operational.
In order to provide modern mobile telephones with extended operating time from their associated batteries, new types of battery have been researched and developed which provide enhanced charge storage to weight performance, for example rechargeable metal hydride and lithium batteries.
The inventor has appreciated, rather than concentrating on improving battery technology, that reduction in current consumption of intermediate frequency amplifier circuits in radio receivers is desirable to provide extended operating time from batteries. The invention has therefore been made in a endeavour to provide an alternative type of amplifier circuit, for example a circuit especially suitable for use at intermediate frequencies in radio receivers which is capable of requiring less power to operate.
It is known in the art, as described in a Japanese patent application no. JP 55137707, to cascade reflection amplifiers in series and interpose filters therebetween to prevent higher harmonic components generated in preceding stages from propagating to successive stages. The filters are not operable to inhibit signal propagation in a reverse direction along the cascaded series of amplifiers to prevent the occurrence of spontaneous oscillation.